The present invention relates generally to a document processing system equipped with user invokable mapping that minimizes the formation of ragged edges around image elements, and more particularly to a method of locating the boundaries of the image elements and mapping their original colors or textures to solid colors.
Color document processing systems typically include an input device such as a computer workstation that is equipped with document applications for creating, editing and storing electronic documents and an output device such as a printing system for rendering documents. The computer workstation is operated by a user to create, edit or view xe2x80x9csoftcopyxe2x80x9d color images on the. color monitor of the workstation. The user can subsequently generate xe2x80x9chardcopyxe2x80x9d reproductions of the softcopy images by instructing the workstation to provide the processed electronic image to a selected color printing device. Current advances in color printing technology are helping color document processing systems become more pervasive in desk-top publishing and business applications. Ink jet, thermal-transfer and xerographic color are examples of printing technologies that are making moderate resolution color affordable for these applications. Although more affordable, these printing technologies have some limitations which may surprise and disappoint a user.
Electronic images processed by the workstation consists of a two dimensional array of picture elements (pixels). The color of each pixel may be represented in any of a variety of color notations or color spaces. The colors of softcopy color images are typically defined using a device dependent color classification space such as the additive red, green and blue (RGB) phosphor color space. More specifically, each pixel of the monitor""s display contains three primary color phosphors. To generate a color defined by a set of RGB values, the monitor stimulates each primary phosphor with an intensity determined by the corresponding R, G, B value. To be printed, these images need to be converted to the subtractive cyan, magenta, yellow and black (CMYK) or (simply the CMY) color space, which is typically used to put colored dyes, inks, or toners on paper.
Document processing systems typically contain predetermined transform definitions for converting an image defined in one color space to another color space. These transformations are typically defined using a look up table (LUT), that enables a color to be readily mapped from one space to another. Accordingly, the color of each pixel of an electronic image is sequentially mapped using a LUT transform definition to yield a hardcopy representation of a softcopy image. To perform other image transformations that perform functions to enhance or sharpen a color, the system remaps the color values to yet another point in accordance with another transform definition. Any number of transformations can thus be performed by sequentially mapping color values according to the available predetermined transform definitions.
Transformations used to convert softcopy images to hardcopy images, however, are limited by the color gamuts afforded to softcopy displays and to hardcopy reproduction systems. For example, because of physical limitations of a printing system, such as its resolution, a softcopy of a color image may lose detail when reproduced as a hardcopy. Since the hardcopy reproduction may not have sufficient resolution to reproduce a softcopy image exactly as represented on a color monitor, the printing system may sacrifice appearance detail of a softcopy image in order to preserve its color fidelity. One instance of this limitation is the production of fine lines and text for certain colors. Unlike CRT displays that tend to have many intermediate color shades, most printing technologies are binary in nature, marking with either full ink or none at all. These printing technologies consequently reproduce intermediate shades and tints with a halftone pattern of solid dots. Thus, when a fine line is drawn using such a dot pattern, gaps or stripes in the line may result. For example, small text may have its boundary so disrupted by a halftone dot pattern that the hardcopy rendering of it may be illegible.
In addition to color printing systems, black-and-white printers that render colored documents using colors that are mapped to certain texture patterns also have a tendency to lose detail when rendering text or fine line graphics specified with a color mapped to a texture. For example, U.S. Pat. No. 5,153,576 to Harrington, provides a halftone-like method for automatically generating texture patterns for image elements by operating on an image, pixel-by-pixel, to produce a pattern for a specified color. Since these generated patterns must have a texture coarse enough to be seen and to be distinguishable from other patterns, the generated patterns have a tendency to introduce some degree of raggedness to the edges of the colored objects that they specify.
Since color printing is performed using a gamut of colors that includes tints and shades of the full color spectrum (e.g. reds, greens, blues and their combinations), printing colored text or fine line graphics on a moderate resolution printing system is difficult for all but a few solid colors. Thus because of physical device limitations, such as a printing system""s resolution, many softcopy color images are inadequately rendered as hardcopy. Specifically, there exists many colors for which lines and text look fine as softcopy on a CRT display but are unacceptable when printed as hardcopy on intermediate resolution printing systems. There exists therefore a need to provide image transformations or mappings that preserve the appearance detail of softcopy images apparent when displayed on color monitors and lost when reproduced as hardcopy on color printing systems. Text, for example, is useless if not legible. Consequently, these transformations should go so far as to sacrifice the color fidelity of the text in order to insure that its appearance detail is preserved.
By way of background, communication protocols between devices such as workstations and printing systems are well known. Some of these protocols define how systems should integrate across networks to provide users with an environment for color document processing. In such environment communication between devices is transparent to users as a result of the various network protocols that define the manner in which devices exchange information. Specifically, document processing systems can be integrated using Ethernet(trademark) and the Xerox Network Systems Communication Protocols which include: Internet Transport Protocols: Xerox System Integration Standard, Xerox Corp., Stamford, Conn., December 1981, XSIS-028112; Courier: The Remote Procedure Call Protocol, Xerox System Integration Standard, Xerox Corp., Stamford, Conn., December 1981, XSIS-038112; Clearinghouse Protocol, Xerox Corp., Stamford, Conn. , April 1984, XSIS-078404; Authentication Protocol, Xerox Corp., Stamford, Conn., April 1984, XSIS-098404; and Filing Protocol, Xerox Corp., Stamford, Conn., May 1986, XNSS-108605.
Also, protocols establishing how to encode electronic documents for transmission between various workstations and printing systems using communications protocols is well known. For example, documents can be encoded using a page description languages (PDL) such as Interpress(trademark) as disclosed in xe2x80x9cInterpress(trademark): The Source Bookxe2x80x9d, Simon and Schuster, Inc., New York, N.Y., 1988, by Harrington and Buckley. In combination with Interpress, the Color Encoding Standard, Xerox System Integration Standard, Xerox Corp, Palo Alto, Calif., July 1991, XNSS 289107 (xe2x80x9cThe Xerox Color Encoding Standardxe2x80x9d), provides a standard for interchanging electronic color documents among document applications and devices. The Xerox Color Encoding Standard describes three reference color systems that attempt to provide device independent color between devices such as workstations and printers.
As described above, many factors, such as resolution, affect the true appearance of an image rendered by different physical devices. Consequently, The Xerox Color Encoding Standard, suggests using xe2x80x9cappearance hintsxe2x80x9d, in addition to a reference color system. Appearance hints provide additional information when describing a color. In particular, one appearance hint provides an ability to indicate that, when reproducing an image, it is more important to a user that the detail of the image is reproduced than its original color fidelity. Co-pending U.S. patent application Ser. No. 08/168,444 entitled xe2x80x9cColor Mapping To Preserve Detailxe2x80x9d, filed Dec. 17, 1993 describes a method, that is prompted by an appearance hint, for preserving detail of an image element by mapping the original color of the image element to an alternative color which appears solid when printed.
All references cited in this specification, and their references, are incorporated by reference herein where appropriate for appropriate teachings of additional or alternative details, features, and/or technical background.
In accordance with one aspect of the present invention there is provided a method for mapping an original color of an image element boundary to a color that appears solid when reproduced. The original color and the color that appears solid being defined by a plurality of color separations. The method includes the steps of locating a section of the image element boundary, and determining whether the image element boundary section located in the locating step forms a true boundary section of the color image. Subsequently, a color separation of the image element boundary section is mapped to a solid color in response to the selection of the image element boundary being a true boundary section. The mapping preserving edge detail in the image element being reproduced.
In accordance with another aspect of the invention there is provided an apparatus for mapping an original color of an image element boundary to a color that appears solid when reproduced. The original color and the color that appears solid being defined by a plurality of color separations, The apparatus includes means for locating a section of the image element boundary, and means for determining whether the image element boundary section located by the locating means forms a true boundary section of the color image. Subsequently, means map a color separation of the image element boundary section to a solid color in response to the selection of the image element boundary being a true boundary section, to preserve edge detail in the image element being reproduced.
In accordance with yet another aspect of the present invention there is provided a method for mapping original colors of image object boundaries forming an image to colors that appear solid when reproduced. An image object being defined by a set of pixels that represent one or more image elements having boundaries with a color defined by a plurality of colorants. The method including the steps of locating a boundary pixel forming an image element, locating a companion pixel adjacent to the boundary pixel and external thereto, calculating a difference value for each colorant defining the boundary pixel and the companion pixel, determining whether the difference value for each colorant exceeds a threshold value, the determining step indicating that the boundary pixel and the companion pixel are true boundary pixels of the image object in response to the difference value of a colorant exceeding the threshold value, and maximizing the larger of the colorants defining, the true boundary pixels.